The invention relates to pipes which comprise an exterior reinforcement layer and an internal multilayer liner which comprises a polyamide layer and a functional layer made of another polymer. These pipes are in particular used for the conveying or transport of crude oil or natural gas or for the transport of supercritical CO2 especially under conditions where relatively rapid pressure changes are likely to occur. Pressure changes of this type are a phenomenon known in the oil and gas industry as “Rapid Gas Decompression” (RGD).
Methods of handling such pressure changes are therefore required in, for example, tertiary mineral oil production. Tertiary oil production uses supercritical CO2 as solvent for residual oil, reducing its viscosity and facilitating extraction from the oil-bearing strata. The critical temperature for CO2 is 31° C., the critical pressure being 73.8 bar. In practice, markedly higher pressures are used, since the solvent power of supercritical CO2 increases with pressure. Typical pressures are in the range from 75 to 220 bar, and the temperature under these conditions can be up to 150° C.
The pipes which transport supercritical CO2 often have a polymeric inliner in order to protect the usually metallic structure from corrosion. In the case of transport pipes, the liner is usually composed of polyethylene; however, it can also be composed of polyamide or of PVDF.
Rapid pressure changes can moreover occur during the operation of crude-oil pipelines and gas pipelines, for example, when the pipeline is depressurized for maintenance operations, with a sudden pressure decrease. A sudden pressure decrease can also occur in such conveying lines, collection pipelines or transport pipelines in the event of an emergency shutdown.
Inliners made of polyethylene exhibit severe swelling in contact with crude oil or natural gas, in particular at the pressures and temperatures used, and this leads to undesired changes in length. This phenomenon can be mitigated if there is a polyamide layer protecting the polyethylene layer on the inside, i.e. on the side facing towards the fluid conveyed. At higher usage temperatures, polyamide is often used as inliner material; it is preferable that this layer is protectively covered on the inside by a polyolefin layer, a fluoropolymer layer or a combination of polyolefin layer and fluoropolymer layer, in order that the polyamide, which is susceptible to hydrolysis at relatively high temperatures, is protected from the water present in the fluid conveyed.
Flexible pipes of multilayer structure with unbonded layers are described in WO 2011/128175 and WO 2011/128197. The pipes described in those documents comprise an inner liner in the form of a multilayer pipe, where the individual layers of said multilayer pipe do not necessarily have adhesive bonding to one another. These pipes can comprise, on the inside of the inner liner, a reinforcement which is usually termed carcase, in order to prevent collapse of the inner liner under high external pressure.
Particularly in the cases where the pipe does not comprise any carcase, it is important that the individual layers of the liner adhere securely to one another. If there is no adhesion, a gas-filled annular space forms between the layers during operation. The gas pressure of the annular space is in equilibrium with the partial pressure of the gas in the conveyed fluid. In the event of pressure variations in the pipeline, the gas located in the annular space can expand; in the worst case, the gas of the annular space can assume a volume which can lead to radial contact between parts of the interior layer of the liner. This leads to blockage of the cross section of the pipe and, in the worst case, prevents any further passage of the fluid conveyed. This type of failure is known to the person skilled in the art as collapse and is the predominant type of failure of inliners. If the gas volume accumulated between the layers is large, the forces arising on expansion of the gas due to pressure reduction in the pipe can also cause collapse of an internal carcase. This type of failure is known to be fatal in the pipes concerned.
However, secure layer adhesion is not a sufficient precondition for preventing collapse of the inner liner layer. It has been found in practice that in many cases where there is initially secure adhesion between a polyamide layer and a polyolefin layer or fluoropolymer layer the adhesion becomes progressively weaker during operation, and the layers finally separate, and the innermost layer can collapse if pressure within the pipe falls.
The object of the invention was to provide a pipe which has exterior reinforcement layer and multilayer liner, and in which a polyamide layer and a polyolefin layer or fluoropolymer layer adhere securely to one another, and in which this secure adhesion is in essence retained during operation.